In recent years, AC-type plasma display devices (AC-PDPs) have been rapidly widespread due to their large screen and slimness compared with conventional cathode-ray-tube televisions and others. However, due to the large screen, its high power consumption and cost becomes problematic.
The display panel of the AC-PDP has X electrodes and Y electrodes alternately arranged approximately in parallel to one another and also has address electrodes crossing in a direction perpendicular to these electrodes to form a two-dimensional matrix.
FIG. 32 is a conceptual drawing of the cell structure of the display panel. A front glass 10a and a rear glass 10f are separated by ribs, and discharge gas, such as Xe, is enclosed in a discharge space 10 therebetween. A Y electrode 9Y and an X electrode 9X are formed in the front glass 10a, and a dielectric layer 10b for insulating from the discharge space 10 is formed thereon. Further, an MgO (magnesium oxide) protective layer 10c is formed thereon.
On the other hand, an address electrode 9A is formed on the rear glass 10f, and a dielectric layer 10e for insulating from the discharge space 10 is formed thereon. Further, a phosphorous layer 10d is formed thereon.
Drive of the plasma display panel can be divided into a reset period of resetting charges accumulated in the cell, an address period of selecting a light-emission position of the panel, and a sustain period of emitting light from the panel and controlling brightness. In the address period, a voltage is applied between the address electrode 9A and the Y electrode 9Y for discharge so that wall charges are added to the cell, whereby, a cell for light emission in the next sustain period can be selected.
Next, the operation in the sustain period for light emission of the plasma display panel is described. When a voltage is applied to the Y electrode 9Y and the X electrode 9X, a voltage is applied to the discharge space 10. When the voltage becomes equal to or larger than a discharge voltage, light emission occurs. That is, in terms of an electric circuit, a switch 9c is turned ON and a discharged state occurs. When this discharge stops, light emission also stops. To repeat light emission, a voltage is required to be applied to the X and Y electrodes of the panel alternately.
FIG. 33 shows changes with time regarding voltages to be applied to the X and Y electrodes of the panel and currents flowing through the panel. The operation for light emission by applying a voltage to a Y side is described below. The same goes for the case where a voltage is applied to an X side for light emission. First, during a period a, an XY wiring capacitance of the panel corresponding to 9d of FIG. 32 is charged to increase a voltage to be applied to the cell. At this time, a charge current flows through the panel. When the voltage to be applied to the cell becomes higher than a firing voltage, the cell emits light, and a gas-discharge current with light emission flows during a period b. At this time, due to an inductance of the wiring, a resistance of a switch element, and other factors, the panel voltage is decreased by ΔV. Next, the panel voltage is decreased in a period c. At this time, a discharge current flows through the panel. This operation of applying a voltage alternately to the X and Y electrodes of the panel for light emission is repeated at a high speed of several ten to several hundred kHz.
FIGS. 34A and 34B show main driving circuits and their operations for achieving the above-described operation. First, a bi-directional switch element 402y is turned ON to charge the XY wiring capacitance of the panel via a coil 5y. This corresponds to the period a in FIG. 33, and a current flows through a path indicated by the charge current shown in FIG. 34A. After the voltage of a Y-side electrode 6y of the panel is increased to a predetermined voltage, when a switch element 1y (hereinafter referred to as a clamp element) is turned ON to clamp the panel voltage at a power-supply voltage Vs, the voltage applied to the cell becomes equal to or larger than the discharge voltage, thereby emitting light. This corresponds to the period b in FIG. 33, and a current flows through a path indicated by the gas-discharge current causing light emission in FIG. 34B. Next, the bi-directional switch element 402y is turned ON to discharge the XY wiring capacitance of the panel via the coil 5y. This corresponds to the period c in FIG. 33, and a current flows through a path indicated by the discharge current in FIG. 34A.
Charging and discharging of the XY wiring capacitance of the panel is performed via the coil because the panel voltage is increased and decreased by using a resonant operation of the XY wiring capacitance and the coil of the panel, thereby reducing a loss in the XY wiring capacitance of the panel at the time of charging and discharging.
In the above-described operation, due to the charge current, the gas-discharge current causing light emission, and the discharge current passing through the switch elements, a loss occurs in each switch element. Such a loss is a cause of increasing power consumption of the plasma display device. Moreover, since a driving circuit is required on both X and Y sides of the panel, the number of components is increased. Such an increase is a cause of increasing cost.
To solve the problems in achieving a low loss of the driving circuit and a reduction in the number of components, driving circuits shown in FIGS. 35 and 37 have been suggested.
FIG. 35 shows a driving circuit of a plasma display device disclosed in Japanese Patent Application Laid-Open Publication No. 2000-330514 (Patent document 1). The driving circuit has a feature in which as a switch element, an Insulated Gate Bipolar Transistor (IGBT) is used in place of a conventional power MOSFET. Unlike the power MOSFET, in the IGBT, conductivity modulation occurs in the element. Therefore, the resistance is small, thereby reducing a loss in the driving circuit.
Here, in FIG. 35, only one driving circuit of the panel is shown, and the other one is fixed to the ground, but the driving circuit side fixed to the ground is considered to be omitted. The reason is that the amplitude of the shown driving circuit is based on a power-supply voltage from the ground, and therefore the voltage applied between X and Y of the panel cannot be changed to be positive or negative. Thus, as the AC-PCP, light emission cannot be repeated. For this reason, the driving circuits suggested in Japanese Patent Application Laid-Open Publication No. 2000-330514 are considered to be as shown in FIG. 36.
However, unlike the power MOSFET, a general IGBT does not incorporate a diode. Therefore, as shown in FIG. 36, when a discharge current flows at one driving circuit, a diode has to be added in inverse-parallel in order to pass a current from an emitter to a collector of an IGBT of the other circuit. For this reason, there are problems of increasing a number of components, complexing circuitry and assembling process, and becoming high cost.
FIG. 37 shows a driving method published in “New Two Stage Recovery (TSR) Driving Method for Low Cost AC Plasma Display Panel”, IDW (International Display Workshops) '05. In this driving method, either one of an X side or a Y side of the panel is fixed to the ground, and positive and negative voltages are alternately applied to the other side, thereby sustaining light emission. This driving method is hereinafter referred to as a half-bridge driving method. By contrast, a method used in the circuitry in FIGS. 34A and 34B is referred to as a full-bridge driving method.
In the half-bridge driving method, unlike the full-bridge driving method, a driving circuit on one side can be omitted, therefore, significantly reducing the number of component is achieved. Furthermore, in FIGS. 34A and 34B, at the time of passing a gas-discharge current with light emission, a charge current, and a discharge current, a voltage drop occurs at switch elements at both of the X and Y sides in the full-bridge driving method. By contrast, advantageously, in the half-bridge driving method, a voltage drop occurs only at the switch element on one side. However, in the half-bridge driving method of FIG. 37, a power MOSFET is used as a switch element, and the present inventors have found through studies that there are problems as described below.
FIG. 38 shows panel driving waveforms in the half-bridge driving method disclosed in the above-described publication. In an AC-PDP, as described above, positive and negative voltages are alternately applied between X and Y of the panel, thereby repeating light emission. Therefore, in the half-bridge driving method in which one side of the panel is fixed to the ground, positive and negative voltages ±Vs are required to be applied to the other side of the panel to drive the panel. In the full-bridge driving method, voltages to be output from the driving circuit are from 0V to Vs, the breakdown voltage of the switch element in the half-bridge driving method is disadvantageously doubled compared with the full-bridge driving method.
Vs of the AC-PDP is on the order of 200V. In the full-bridge driving method, as a switch element, a power MOSFET with a breakdown voltage on the order of 300V is used. Therefore, the power MOSFET for use in the half-bridge driving method is required to have a breakdown voltage on the order of 600V.
Output characteristics of these power MOSFET are shown in FIG. 39. From these results, it can be found that a resistance of one 600V power MOSFET is larger than a resistance of two 300V power MOSFET in series with a full-bridge driving method. This is because the resistance of the power MOSFET is increased in proportion to 2.5-th power of the breakdown voltage.
For this reason, in the half-bridge driving method using the power MOSFET of FIG. 37, there has been a problem that a loss of the driving circuit is disadvantageously increased compared with the full-bridge driving method using the power MOSFET.